Millions of dollars have been spent trying to figure out what makes the popping sound in a joint. Science has measured out how far and how fast a joint has to be moved to get the "audible release".
However, how the sound is caused is only a theory. It is speculated that when a joint is moved a certain speed and distance, an air bubble forms out of the synovial fluid (the lubricant within the joint), causing the sound.
Based on his own understanding of physics, Dr. Chew is skeptical of this theory for several reasons.
1) When air bubbles gets in the joints of a scuba diver who doesn't properly decompress, it's painful, and he/she gets the bends.
2) Gas going from liquid state into gaseous state due to decompression does so uniformly throughout the liquid as can be seen when a carbonated beverage is opened.
3) The transition out of liquid occurs gradually whereas the sound of an adjustment or cracking ones knuckles is insantaneous.
In Dr. Chew's opinion, the noise is caused by the sudden pressure change in the synovial fluid just as if you popped a balloon in a confined space.
This is Dr. Chew's opinion and is not based on any published research.
Dr. Chew's favorite study regarding this topic compared knuckle crackers to people who didn't crack their knuckles. Researchers found that on physical examination people who crack their knuckles have bigger knuckles but x-ray examinations reveal less arthritis in the people who crack their knuckles.
The theory is that moving a joint beyond its normal range of motion on a regular basis causes joint capsule thickening which is actually strengthening the joint and helps pull water in and out which helps the joints nutrition.
Anecdotally, in his early years in practice, a young man sought Dr. Chews advice. The young man was concerned that he was causing himself damage because he was always cracking his neck. In the course of a three minute interview, he was observed to crack his neck about 20 times by grabbing his neck and twisting his head. On inspection, the area that he grabbed was enlarged and actually discolored.
Dr. Chew told the young man that he couldn't be sure what, if any, damage was being done without an x-ray. The young man asked about the cost of an x-ray and said that he would be back that afternoon. Unfortunately, he never returned.
To this day, Dr. Chew wonders what that x-ray would have shown.
It is the general opinion of most chiropractors that a person who feels the need to pop their own joints needs professional chiropractic treatment.